Here's an article announcing new cancer-only policies by Pets Best.
Most people would be better served buying more comprehensive accident and illness coverages that also includes coverage for cancer. However when I interviewed Pets Best Founder and President Dr. Jack Stephens for a podcast episode, he alluded to their research that some pet owners were more concerned about cancer than any other illness their dog or cat might experience, so that's the reason for the cancer only policy.
They are in the process of getting several new policies including the cancer-only policy approved in all 50 states, so all of them may or may not be available in your state yet. You can get a quote to find out which policies are available for your pet.